1. Field of the Invention
The present invention pertains generally to the manipulation of drill pipe and drill collars in a welldrilling derrick. More specifically, the invention pertains to a derrick mounted, maneuverable racker arm for picking up and relocating essentially vertically oriented drill pipe and drill collars.
2. Description of the Prior Art
In the drilling of oil and gas wells, the drill string is made up of pipe segments commonly stored upright within the derrick. These pipe segments, usually assembled in groups of three to form a "stand," are picked up by conventional hoist means mounted in the derrick and successively screwed into the string of pipe already suspended in the well bore. In withdrawing the drill string, the procedure is reversed with the stands being unscrewed from the suspended string as the string is withdrawn from the well, and returned to the vertical storage position. Conventionally, these operations require considerable manual labor and expenditure of time, particularly in making a so-called "round trip"in which the entire drill string is withdrawn from the well to change a bit, or for other purposes, and then returned to the bottom of the well.
Electrically or hydraulically powered pipe handling systems have been developed to transport pipe members between a storage area within the derrick and the well drilling location. The objects of such systems are to reduce the manual labor required in such operations and to speed up the entire pipe handling process. These systems generally feature one or more movable arm mechanisms equipped with some means of engaging the pipe while maintaining the pipe in an essentially vertical orientation. Once engaged, the pipe may be taken from a storage area and held over the well position. At that point, a conventional pipe supporting mechanism is attached, such as an elevator, the pipe is connected to the string suspended in the well, and the arm mechanism is disengaged and withdrawn. The steps are reversed when pipe is to be withdrawn from the well, disconnected from the string, and placed in the storage area.
Whenever pipe is thus maneuvered within a derrick, it must be lifted and supported off of the derrick floor to clear the wellhead structure, etc., and to be set down on a setback structure - the platform on which the vertical standing pipes are stored. This vertical movement is usually supplied by a cable attached to the pipe-engaging means and passing over a sheave, mounted in the derrick, down to a power unit. Provision is made for the pipe engaging means to be movable vertically with respect to the arm, and the power unit operates to raise or lower the pipe engaging means as needed. If the pipe engaging means for lift is in the form of an elevator, suspended by the cable free of the arm, such an elevator must be manually placed on the pipe.
In some cases, the pipe is engaged for lifting purposes by a two-prong device which fits under the threaded box end of the pipe. Then, the maneuvering of the arm cable mechanism to pick up pipe can be a delicate operation.